1. Field of the Invention
The present invention relates to a motor control apparatus which converts AC power supplied from an AC power supply into DC power, outputs the DC power to a DC link, further converts the DC power into AC power for driving a motor, and supplies the AC power to the motor, a controller for a converter used for the motor control apparatus, and a machine learning apparatus and method for the controller for a converter.
2. Description of the Related Art
In a motor control apparatus which drives motors within a machine tool, a forging machine, an injection molding machine, an industrial machine, or various robots, AC power supplied from an AC power supply is temporarily converted into DC power, which is further converted into AC power. The AC power is used as drive power for a motor provided for each drive shaft.
FIG. 7 is a block diagram illustrating the configuration of a general motor control apparatus. A motor control apparatus 100 includes a converter 101 and an inverter 102. The converter 101 converts AC power from a commercial AC power supply (to be simply referred to as an “AC power supply” hereinafter) 103 into DC power. The inverter 102 converts DC power output from the converter 101 into AC power having desired frequencies and supplied as drive power for a motor 104 or converts AC power regenerated from the motor 104 into DC power. The motor control apparatus 100 controls the velocity, torque, or rotor position of the motor 104 connected to the AC terminal of the inverter 102. For the sake of illustrative simplicity, FIG. 7 illustrates driving of only one motor 104. In general, when a plurality of drive shafts are driven, a motor is provided for each drive shaft. In this case, a plurality of inverters are used to independently supply drive power to each motor.
The converter 101 is connected to the inverter 102 via a DC link (Direct-Current link). A DC link capacitor 105 is provided in the DC link. The DC link capacitor 105 has a storage function for storing DC power and a smoothing function for suppressing pulsation of the DC output of the converter 101.
In response to the recent demand for energy saving, as a converter for a motor control apparatus, a power supply regeneration converter which can return regenerative power generated upon motor deceleration to an AC power supply, as disclosed in, e.g., Japanese Patent No. 2567830, is widely used.
A power supply regeneration converter has a function for converting AC power supplied from an AC power supply into DC power and outputting the DC power to a DC link, as well as a function for converting DC power supplied from the DC link into AC power and outputting the AC power to the AC power supply, upon motor deceleration. Regenerative power generated upon motor deceleration is converted from AC power into DC power by an inverter. The DC power is input to the converter via the DC link and further converted by the converter into AC power, which is supplied to the AC power supply for power supply regeneration.
Examples of the power supply regeneration converter may include a PWM-controlled rectifying circuit and a 120-degree conduction rectifying circuit.
Of these examples, the PWM-controlled rectifying circuit is implemented in a bridge circuit consisting of a semiconductor switching element and a diode connected in inverse parallel to it, as disclosed in, e.g., Japanese Unexamined Patent Publication No. H8-47279. PWM switching control of an internal semiconductor switching element of this circuit is performed to convert power between AC power in the AC power supply and DC power in the DC link.
In such a motor control apparatus, the DC voltage applied across the two terminals of a DC link capacitor provided in the DC link (to be sometimes simply referred to as the “DC link voltage” hereinafter) varies depending on the amount of drive power consumed by a motor or the amount of regenerative power generated by the motor, the amount of power conversion from AC power into DC power or from DC power into AC power by a converter, and the amount of power conversion from DC power into AC power or from AC power into DC power by an inverter. At the time of, e.g., motor deceleration, AC regenerative power generated by the motor is converted into DC power by the inverter. A conversion operation from DC power into AC power by the converter (to be referred to as “a power supply regeneration operation by the converter” hereinafter) is adjusted to, in turn, adjust the DC link voltage to a voltage that does not break down the DC link capacitor and respective elements in the converter and the inverter.
Several methods for controlling a power supply regeneration operation by the converter in accordance with the DC link voltage have been proposed, as disclosed in, e.g., Japanese Patent No. 5319318.
As described above, AC regenerative power generated by the motor upon motor deceleration is converted into DC power by the inverter and the DC power is returned to the DC link, so that the DC link voltage rises. When the regenerative power raises the DC link voltage too much, the DC link capacitor and respective elements in the converter and the inverter break down, or the motor control apparatus stops due to an overvoltage alarm set to prevent breakdown. Therefore, the converter preferably performs a power supply regeneration operation to return energy in the DC link to the AC power supply. On the other hand, power supply regeneration by the converter causes a switching loss, leading to heat generation in the semiconductor switching element in the converter. Especially when power supply regeneration is performed for a long time, the semiconductor switching element in the converter generates heat in large quantities. Then, the semiconductor switching element in the converter suffers thermal breakdown or the motor control apparatus stops due to an overload alarm set for protection of the semiconductor switching element in the converter from overheat. For this reason, it is very important to perform adjustment for an efficient, appropriate power supply regeneration operation by the converter so that the DC link voltage does not rise too much, while minimizing the execution of power supply regeneration. Such adjustment is performed by the designer in the design stage of a motor control apparatus or by the operator in the adjustment stage of a machine equipped with the motor control apparatus. It is troublesome to manually perform such adjustment in accordance with the operating states of individual machines.